The concept of collecting light from a central lighting source, introducing it into a light guide such as an optical fiber, and channeling it through the guide to a remote location has been proposed for various applications such as automotive, display lighting, and home lighting. U.S. Pat. No. 4,958,263 issued to Davenport et al. on Sep. 18, 1990, which is assigned to the same assignee as the present invention describes the use of this concept in a central lighting system for automotive applications. The goal of this automotive central lighting system as well as any other central lighting system is to achieve the most efficient light output at the point of delivery and deliver such light output in a manner that allows for the specific lighting design considerations. For instance, future designs in forward lighting systems for automobiles are tending towards sleek, narrow head lamp designs for both improved aerodynamic performance of the vehicle and aesthetic appeal. These designs require a small narrow beam of light to provide the necessary illumination. In order to provide a narrow beam, as is required for this specific application as well as others, small diameter fibers on the order of 6 to 8 mm are used for delivering the light. An elliptical reflector is typically used for collecting light from the light source and focusing it onto the entrance face of the optical fiber. In known systems, the light source is placed at the first focal point of the ellipse closer to the elliptical reflector surface. The elliptical reflector forms an image of the light source at its second focal point, wherein both the first and second foci are located on the axis of the reflector. The image is magnified by a factor of four to five times. It is also known that the input end of the optical fiber is disposed at the point of formation of the magnified image so that the beam of light enters the input face of the optical fiber at a full angle of 60 degrees, which in turn necessitates that the light exit the output end of the fiber at the same full angle of 60 degrees. In this manner a narrow beam of light suitable for use in automotive applications is produced. The four to five fold magnification of the image, however, imposes a limitation on the minimum diameter fiber that can be used, since a somewhat larger diameter fiber must be used in order to collect all the light from the magnified image. In other words, the diameter of the optical guide must be on the order of four to five times the size of the light source to collect substantially all the light from the image.
It would be advantageous to use the smallest possible diameter fiber since a smaller diameter fiber would provide the benefits of cost, size, weight, and design flexibility, in addition to providing a more controlled beam pattern. In addition, a smaller diameter fiber has a higher lumen density, measured as the lumens per unit cross sectional area of the optical fiber. In order to provide a most efficient light delivery system substantially all the light from the image must be introduced into the input face of the fiber. To achieve such an efficient light delivery system using a smaller diameter fiber would require that the magnification of the image formed by the reflector be reduced, preferably to unit magnification. In this case an optical fiber of the same order of magnitude as the light source can be used to collect the light from the light source in an efficient manner. It is known in the prior art that unit magnification of the light source can be achieved by using a spherical reflector in place of an elliptical one. In fact, U.S. Pat. No. 4,956,759 describes an illumination system that employs a spherical reflector to form a unit magnification image. In such a system the light source is positioned near the center of curvature of the reflector and an image of unit magnification is formed at a point near the source. In order to collect the maximum amount of light from the light source the input face of the optical fiber would have to be placed at the point of formation of the unit magnification image disposed near the center of curvature of the reflector. With this arrangement the light entering the input face of the optical fiber from the reflector would do so over a solid angle of up to 180 degrees. The fact that the beam of light emerges from the output end of the fiber at the same angle of up to 180 degrees that it entered the fiber presents the problem that the beam is too wide for most applications that require a narrow controlled beam of light. This invention provides a method for reducing the angular divergence of a beam of light exiting such optical fiber from what it was entering the fiber so that the light output is useable for automotive and other applications that require a narrow beam of light.
Non-imaging optical couplers may be used to reduce the angular divergence of the beam into a narrow controlled beam. Such couplers have been described to a great extent by R. Winston and W. T. Welford in The Optics of Nonimaging Concentrators and Solar Energy. Academic Press, 1978. Additionally, previously referenced U.S. Pat. No. 4,956,759 provides an illumination system with two spherical reflectors for coupling light from a light source to form an image and a non-imaging coupler to focus the light from the image into a controlled beam. While the non-imaging coupler can perform this task, it does so using reflection at the surface of the non-imaging coupler. The non-imaging coupler is unsuited for this application because of the bulky nature of the non-imaging reflector. A bulky system would be particularly disadvantageous in applications that require reduced size and weight as well as design flexibility and ease of manipulation of the light source.
It is the objective of this invention to provide a lens for reducing the angular divergence of a beam of light emerging from an optical fiber of minimum diameter into a narrow controlled beam and to achieve the narrow beam in an efficient illumination system that lends itself to reduced size, weight, and cost and in which the light beam can easily be manipulated in a precise manner.